This invention relates to aircraft turbojet and turbofan engines and particularly to means for suppressing jet noise occasioned by the hot gases discharging from the tail pipe.
As is well known the exhaust gases discharging from the tail pipe of a gas turbine type power plant, particularly during the take-off regime, has been a source of unwanted jet noise caused by the mixing of the high velocity exhaust gases with the surrounding ambient air. Many types of suppressors have been developed to reduce this jet noise, but most have economic disadvantages. Fixed geometry suppressors can provide some noise reduction at take-off, but they invariably cause a loss of thrust at cruise, thereby increasing fuel consumption over the entire operating range of the engine. Variable geometry suppressor designs shown in the literature are generally too complex, leading to high initial cost and questionable reliability.
This invention contemplates a solution to the same problem but does so in a less complex and hence simpler, less costly manner. A single set of retractable flaps mounted in a particular location from where is heretofore shown, serves as an effective way of reducing noise. I have found that by using an ejector in combination with retractable flaps and judiciously locating the flaps relative to the ejector and providing effective mixing between the engine exhaust gases and the ejector secondary flow, a reduced velocity at the ejector exit will be realized resulting in reduced jet noise.
Additionally, loss of engine thrust during deployment of the flaps can be minimized by fairing the flaps on the downstream side to reduce their drag. In configurations utilizing flaps that pivot radially outward with respect to the tail pipe axis, this invention contemplates the use of an axially moveable plug centrally mounted which forms an annular nozzle in conjunction with the tail pipe exit. This variable area nozzle permits the suppressor to be utilized at various engine conditions other than full take-off thrust with the engine correctly matched via the proper discharge area. For a simpler device which would operate only at full take-off thrust, the moveable plug can be positioned axially for correct enging match and fixed at that location. This would eliminate adjusting the individual flap positions and/or size of the tail pipe openings when the suppressor is installed on the engine.